For a digital camera, a digital video and the like, a CCD (Charge Coupled Device) image sensor, a CMOS (Complementary Metal Oxide Semiconductor) image sensor and the like (hereinafter, referred to as solid-state imaging devices) are used. However, spectral characteristics of the solid-state imaging devices have high sensitivities with respect to infrared light as compared to human visibility characteristics. Accordingly, spectral correction with the use of a near-infrared cut filter is performed in the digital camera, the digital video and the like.
As the near-infrared cut filter, for example, a near-infrared absorption type color glass filter such as fluorophosphate-based glass containing Cu2+ ions as a coloring component has been used. However, since the light in a near-infrared region and an ultraviolet region cannot be sufficiently cut by the color glass filter itself, an optical multilayer having characteristics capable of cutting near-infrared light is used in combination at present.
When the optical multilayer is used in combination, a half-value wavelength on an ultraviolet side of a transmission band through which visible light is transmitted, is determined by characteristics of the optical multilayer, and a half-value wavelength on an infrared side of the transmission band is determined by characteristics of the color glass filter. This is because, since there is a tendency that a spectral waveform of the optical multilayer is shifted to the ultraviolet region side in accordance with an increase in an incident angle of light, and a spectral waveform of the color glass filter is difficult to be shifted to the ultraviolet region side when the incident angle of light is increased, it is reasonable to cut, with the use of the optical multilayer, the light in the wavelength region which cannot be cut by the color glass filter, while utilizing the spectral waveform of the color glass filter as much as possible.
Incidentally, the digital camera, the digital video and the like have been reduced in size and thickness in recent years. For this reason, a lens of the digital camera, the digital video and the like is increasingly made to have a wider angle. In accordance with this, an incident angle dependence of the wavelength region cut by the optical multilayer becomes a problem. Concretely, in a transmission band of 400 to 700 nm required by a solid-state imaging device, a rising position of a transmittance with respect to the transmission band from a stop band on the ultraviolet side and a rising position of a transmittance with respect to the transmission band from a stop band on the near-infrared side formed by the optical multilayer are displaced depending on an incident angle of light, which changes an amount of light in a band (transmission band) which exerts an influence on an image quality.
As described above, it has been conventionally known that the optical multilayer has the incident angle dependence such that the spectral waveform is shifted to the ultraviolet side when the incident angle of light is increased. The incident angle dependence has been a big problem in a field of dichroic mirror and the like used for a prism and the like, so that techniques for suppressing the incident angle dependence of the optical multilayer have been proposed.